In a typical method of encoding a multi-channel audio signal, a multi-channel audio signal is down-mixed into a mono or stereo signal and the mono or stereo signal is encoded together with spatial information, instead of encoding each channel of the multi-channel audio signal. Here, the spatial information is used to restore the original multi-channel audio signal.
FIG. 1 is a block diagram of a typical system for encoding/decoding a multi-channel audio signal. Referring to FIG. 1, an audio signal encoder includes a down-mix module which generates a down-mix signal by down-mixing a multi-channel audio signal into a stereo or mono signal, and a spatial parameter estimation module which generates spatial information. The system may receive an artistic down-mix signal that is processed externally, instead of generating a down-mix signal. An audio signal decoder interprets the spatial information generated by the spatial parameter estimation module, and restores the original multi-channel audio signal based on the results of the interpretation. However, during the generation of a down-mix signal by the audio signal encoder or during the generation of an artistic down-mix signal, signal level attenuation is likely to occur in the process of adding up different channel signals. For example, in the case of adding up two channels respectively having levels L1 and L2, the two channels do not overlap but offset each other so that a level DL12 of a channel obtained by the addition is lower than the sum of L1 and L2.
Attenuation of the level of a down-mix signal may cause signal distortion during a decoding operation. For example, the relationship between the levels of channels can be determined based on Channel Level Difference (CLD) information, which is a type of spatial information and indicates the difference between the levels of channels. However, when the level of a down-mix signal obtained by adding up the channels is attenuated, the level of a down-mix signal obtained by decoding is lower than the level of the original down-mix signal.
As a result of the aforementioned phenomenon, a multi-channel audio signal obtained by decoding may be boosted or suppressed at a predetermined frequency, thereby causing deterioration of the quality of sound. In addition, since the degree of attenuation of the level of a signal caused by a partial offset of the signal by another signal varies from one frequency domain to another, the degree of distortion of a signal after passing the signal through an audio encoder and an audio decoder also varies from one frequency to another. This problem cannot be fully addressed by varying the energy level of a down-mix signal in a predetermined frequency domain.